The present invention relates to a process for recovering argon, and more particularly to an economical process for improving the yield of argon with relatively low consumption of electric power.
In the widely adopted process for recovering argon as shown in FIG. 1, argon-rich oxygen or argon feed gas is extracted from the middle stage of an upper column 4 of a double rectification column 2 of an air separation plant through a conduit 6 and then introduced into a crude argon column 8, where it is cooled and rectified with a liquefied air which is fed from the sump of a lower column 10 via conduits 11 and 12 and a heat exchanger 14, where the liquefied air is sujected to heat exchange with nitrogen gas led from the head of the upper column 4 via conduit 16, heat exchanger 28 and conduit 18. As a result, crude argon issues from the head of the argon column 8 through a conduit 20, and is sent to an argon purifying process (not shown) including a deoxidation unit and a high purity argon column where high purity argon is recovered. On the other hand, the liquefied air fed to a condenser 22 of the argon column 8 is evaporated in that condenser 22 and then passed through a conduit 24 into an upper column 4. Liquified oxygen issues through a conduit 26 from a sump of the argon column 8, and returns to the upper column 4. From the head of the upper column 4 there issues nitrogen gas, which is cooled by liquid nitrogen led through a conduit 17 from the head of the lower column 10 at a heat exchanger 28, and which then passes through conduit 18, heat exchanger 14 and conduit 30, into a heat exchanger 32 where compressed air is cooled by heat exchange with the nitrogen gas and then introduced into the bottom portion of the lower column 10 through a conduit 31.
In recovering argon by means of the air separation plant according to the above-mentioned air rectification process, it is, as described above, a common practice to cool the condenser 22 of the crude argon column 8 by heat exchange with part of the liquid air from the sump of the lower column 10 although the total amount of the liquid air should be supplied in a liquid state to the upper column 4 via a conduit 34. Therefore, in order to enhance the yield of crude argon in the crude argon column 8, it is naturally necessary to increase the amount of liquid air fed from the sump of the lower column 10 via conduit 11, heat exchanger 14 and conduit 12 for cooling the condenser 22, with the result that the supply of the liquid air to the upper column 4 is decreased. Consequently, the rectification performance of the upper column 4 is adversely affected to the point where, in the worst case, no argon feed can be obtained. Thus, the prior art process has an upper limit in the yield of argon. Particularly, in the overall low pressure air separation plant as shown in FIG. 1, output gaseous air of an expansion turbine 36 for generating make-up refrigeration is introduced into the upper column 4 and this air from turbine 36 deteriorates the conditions of rectification, so that the recovery of argon becomes difficult. Furthermore, in an air separation plant of a type that includes recovery of liquid nitrogen, a larger amount of the turbine air is necessary for recovering nitrogen in a liquid state, resulting in deterioration of the rectification conditions. Thus, in such prior art air separation plants a larger proportion of argon contained in the air feed from conduit 31 must be discharged together with nitrogen through conduit 40.
In order to overcome such a drawback, there is a well-known process involving the nitrogen cycle in which the nitrogen gas from the head of the upper column 4 is compressed to a pressure in the lower column 10 and then supplied to the latter after cooling or liquefaction thereof to restore the normal conditions of the rectification in the upper column 4 to thereby enhance the yield of argon. This process is more specifically illustrated in FIG. 2. The nitrogen gas from the head of the upper column 4 is passed via a conduit 42 to a heat exchanger 44 for heating it to around normal temperatures, and then introduced through a conduit 46 into a compressor 48 where it is compressed to a pressure of 4.8 kg/cm.sup.2 G. After the compressed nitrogen gas is cooled to -173.degree. C. at the heat exchanger 44 by heat exchange with the low temperature nitrogen gas from the conduit 42, it is introduced through a conduit 50 to the upper part of the lower column 10, so that liquid nitrogen, which is introduced from the head of lower column 10 through conduit 17 into upper column 4, increases. This increase in the amount of the reflux restores good rectification conditions in the upper column. Although this process with the nitrogen cycle can improve the rectification conditions in the upper column 4, it requires a considerable amount of electric power for compressing the nitrogen gas from atmospheres pressure to a pressure of about 5 atmosphere in the lower column 10. Furthermore, the use of nitrogen gas, which has a small latent heat when liquefied, makes the amount of the nitrogen gas to be cycled larger. This fact further increases the necessary electric power.